I would look at a local salvage yard. If there is a pull-a-part near you then you might be set. Then you could just pull one off of another 1999. You might want to measure, but I am pretty sure that the plates from a z71 would fit. It might require some drilling and tapping for mounting if the holes are not already there. Alternatively, you can check out GMpartsclub.com here are some prices I came across for a 2001 truck. They will not fit yours, but it should give you an idea. The prices are not that bad.
GM PART # 15843753 Aluminum Skid plate protecting oil pan
GM LIST: $39.24